In known wireless cellular telecommunication networks, an area covered by a network is divided into a plurality of cells. Each of these cells has a base station which is arranged to transmit signals to and receive signals from mobile stations located in the cell associated with the respective base station. Mobile stations will be in active communication with the base station associated with the cell in which the mobile station is located.
A number of different cellular telecommunication standards are used or have been proposed. One such standard is the GSM (Global System for Mobile Communications) standard. GSM uses a frequency/time division multiple access technique. A mobile station is allocated a given frequency band and time slots in order to transmit to the base station. The mobile station is allocated a different frequency band and time slots to receive signals from the base station. The allocated frequency band and time slots can be changed over time.
There are a number of different frequency bands at which GSM can be used. For example, one frequency is around 900 MHz. This will be referred to as GSM 900. Another frequency is around 1800 MHz This will be referred to as GSM 1800.
A third generation standard has been proposed and is referred to as UMTS (Universal Mobile Telecommunications System). UMTS uses a code division multiple access technique. With the proposed UMTS system, soft handoff has been proposed. With soft handoff, a mobile station is in communication with more than one base station at the same time.
It is possible that networks using different standards can cover the same area. As the different standards use different methods of access and/or different frequencies, the different networks do not interfere. Multiband mobile phones operate on more than one frequency band of the same standard. Dual mode mobile phones have been proposed particularly in the context of the GSM and UMTS standards. Accordingly, one dual mode mobile station is able to communicate with a network using the GSM standard and a network using the UMTS standard. A multiband mobile station is able to communicate e.g. either on a GSM 900 network or on a GSM 1800 network. At any one time the mobile station will be connected to only one network. In the GSM standard, it is stated that when a mobile station completes a connection and is in an idle mode, the mobile station camps on the cell whose channel has been released. In other words, the mobile station remains associated, in the idle mode, with the cell with which it has just completed a connection. This is sometimes disadvantageous in that the cell with which the mobile station is associated may not be the most appropriate cell. In order to change the cell with which the mobile station is associated or camping on, a cell reselection procedure has to be carried out. This can be a complicated procedure especially if the mobile station is changing from one network to another. A network operator requires extensive experience in parameter setting.
In the GSM standard, parameters are defined for the cell re-selection procedure. With these parameters, which are set by the operator and broadcast on the BCCH/PBCCH of the cell, the mobile station is able to find out, which cell should be considered as the best cell and whether a cell re-selection shall be proceed.
These parameters are, for example, offsets to the measured signal strengths of the serving cell and neighbour cells to get the mobile station to camp on the wanted cell. The wanted cell may also belong to a different location area compared to the cell where the connection was released. Assuming that the call was initiated in one network (e.g. GSM 900) but the mobile station was handed over to another network (e.g. UMTS), typically the mobile station should perform a location update procedure on the cell where the call was released. If then the operator prefers mobile stations to camp on the GSM system in idle mode, then the mobile station would reselect another cell from the GSM network and would again typically perform a location update. This slows down the reselection of the intended cell and adds unnecessary signalling.